Method of controlling loss of a subterranean treatment fluid

ABSTRACT

Selectively cross-linked starches are disclosed that are useful as fluid loss control additives in subterranean treatment fluids comprising starches that cross-linked to a Brabender peak viscosity of about 800 to about 1250 Brabender units after about 40 to about 70 minutes at about 92° C. and provide good fluid loss control over a temperature range of from about 20° C. to about 160° C. (68° F. to 320° F.).

This application is a continuation of U.S. patent application Ser. No.08/901,805, filed Jul. 28, 1997, now U.S. Pat. No. 6,180,571.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to cross-linked starches that are useful as fluidloss control additives for aqueous-based subterranean treatment fluids,such as drilling, workover and completion fluids.

2. Related Background Art

The cross-linked starches of this invention may be advantageously usedin oil field applications. Particularly, the starches may beincorporated into fluids used in operations where there is contact witha subterranean formation. Drilling, workover, and completion fluids areexamples of fluids used in subterranean formations.

Drilling fluids may be used for any of several functions that allowevaluating or producing a reservoir (formation) for oil, gas, or water.The drilling fluid may be pumped into the wellbore during the drillingoperation to cool the drill bit and to flush out the rock particles thatare sheared off by the drill bit. A “drill-in” fluid is often used whiledrilling the production zone.

Workover fluids may be used to perform one or more of a variety ofremedial operations on a producing oil well with the intention ofrestoring or increasing production. Examples of workover operationsinclude, but are not limited to, deepening, plugging back, pulling andresetting a liner, squeeze cementing, shooting and acidizing.

Completion fluids may be used to perform one or more of a variety of oilfield applications illustrated by, but not limited to, operations suchas cementing, using spacers, perforating, gravel packing, installingcasing, underreaming, milling and a variety of simulation techniquessuch as acidizing and the like.

Subterranean treatment fluids are used in well operations, particularlyoil well operations, for various purposes. The subterranean treatmentfluids are generally prepared at the well site by admixing aviscosifying agent and a base fluid. The viscosifying agent thickens orviscosifies the base fluid, thereby increasing the ability of the fluidto suspend or flush out the rock particles. The subterranean treatmentfluid may also advantageously contain other additives that areconventionally used in well treatment operations, as needed, based uponthe specific site requirements and environmental conditions.

A common problem associated with the use of subterranean treatmentfluids is the loss of fluid into the surrounding formation near thewellbore. Fluid loss control additives are added to the subterraneantreatment fluids to limit exposure of the formation and also controlleak off of the liquid components to the surrounding subterraneanformation. As a result, the subterranean treatment fluids that are mostuseful in well operations possess adequate high water retentioncapacity. Desirably, the subterranean treatment fluid should retain highwater retention capacity under the often adverse environmentsencountered during use. For example, high temperature conditions areencountered in deep wells, where operating temperatures frequentlyexceed 250° F. Low temperature conditions are encountered in shallowwells or in areas of a well that are closer to the earth's surface. Highsalt conditions are created when brine-containing subterranean treatmentfluids are used. Accordingly, the fluid loss control additive used insubterranean treatment fluids should preferably be stable in both hightemperature and high salinity environments. More preferably, the fluidloss control additive should be stable over a range of temperatures andshould function in environments of either high or low salinity.

Natural starches are a well known and important class of materialsuseful as fluid loss control additives. However, it is also well knownthat starches do not possess long term stability and tend to degradewhen maintained at elevated temperatures. For example, at temperaturesin excess of 225° F., natural or conventional starches begin to degrade,and will fail to provide adequate fluid loss control.

Several approaches have been used to increase the stability of starchesto provide more stable well drilling fluids. For example, U.S. Pat. No.4,090,968 discloses the use of quaternary ammonium starch derivatives asfluid control additives that are stable at high temperatures. Thesederivatives were prepared by reaction of starch with epichlorohydrin anda tertiary amine.

A thixotropic three-component well drilling fluid, consisting of across-linked potato starch, a heteropolysaccharide derived from acarbohydrate by bacteria of the genus Xanthomonas, and hydroxyethylcellulose, providing improved water loss control is disclosed in U.S.Pat. No. 4,422,947.

U.S. Pat. No. 4,652,384 discloses the use of selected cross-linkedstarches to provide fluid loss control at elevated temperatures. Thestarch, which is cross-linked to a rather high degree under specifiedconditions, requires activation at elevated temperature for over fourhours in order to achieve suitable effectiveness.

Other well treating fluid blends have been prepared by incorporatingXanthomonas gum and an epichlorohydrin cross-linked hydroxypropylstarch, as described in U.S. Pat. No. 4,822,500. This particularcombination of additives interact synergistically to enhance suspensioncharacteristics and decrease fluid loss.

U.S. Pat. No. 5,009,267 discloses fluid loss control additives forfracturing fluids composed of blends of two or more modified, orcross-linked, starches or blends of one or more natural starches withone or more modified starches.

Although many of the cross-linked starch compositions described aboveoffer improvements over conventional starch, there remains a need in theindustry for a readily dispersible starch additive that can provide goodfluid loss control over a wide temperature range and that is stable inbrine-containing fluids.

SUMMARY OF THE INVENTION

This invention is directed to selectively cross-linked starches andblends of these cross-linked starches that are useful as fluid losscontrol additives that provide good fluid loss control over a widetemperature range. More particularly, this invention is directed tofluid loss control additives for use in subterranean treatment fluidscomprising starches which are cross-linked and have a Brabender peakviscosity of about 800 to about 1250 Brabender units after about 40 toabout 70 minutes at about 92° C. (198° F.) and provides good fluid losscontrol over a wide temperature range of from about 20° C. to about 160°C. (68° F. to 320° F.). This invention is also directed to theselectively cross-linked starches that are spray-dried to furtherimprove the starch properties. Additionally, this invention coverssubterranean treatment fluids containing the defined cross-linkedstarches.

DETAILED DESCRIPTION OF THE INVENTION

In this invention, the ability to provide a fluid loss control additivewhich is effective over a wide temperature range by using a selectivelycross-linked starch is demonstrated. This result is surprising andunexpected as evidenced by a review of the literature and commerciallyavailable products which show the use of various starches and modifiedstarches, none of which suggest the particular starches of thisinvention or the degree of fluid loss control exhibited over an extendedtemperature range.

An important feature of this invention is the amount of cross-linkingthat the starch receives, i.e. the amount of treatment or the degree ofcross-linking. While it is difficult to measure this characteristic ofthe treated starch, particularly at low levels, one of the best ways todetermine the amount of cross-linking is to measure the viscosity of thestarch. It is well known to measure the viscosity of cross-linked starchusing a C. W. Brabender Visco-Amylo Graph. Using this measuring deviceand method, the starches of this invention are cross-linked to provide aBrabender peak viscosity of about 800 to about 1250, preferably about920 to about 1150 Brabender units after about 40 to about 70 minutes atabout 92° C. The test procedure for measuring this feature is providedbelow.

The cross-linked starches used in this invention may include starchtreated with a number of multi-functional cross-linking agents. Moreparticularly, the cross-linking agents used in this invention includeepichlorohydrin, phosphorus oxychloride, adipic-acetic anhydrides andsodium trimetaphosphate. Epichlorohydrin and phosphorus oxychloride arepreferred cross-linking agents and epichlorohydrin is most preferred.

The starches which may be used as the base material in preparing thecross-linked starch of this invention may be derived from any plantsource including corn, potato, wheat, rice, sago, tapioca, waxy maize,waxy rice, and sorghum. Also useful are the conversion products derivedfrom any of the above base materials including, oxidized starches,prepared by treatment with oxidants such as sodium hypochlorite, andfluidity or thin-boiling starches, prepared by enzyme conversions ormild acid hydrolysis. Preferred starches are corn, waxy maize, potato,wheat and tapioca, with waxy maize being especially preferred.

The cross-linked starches of the present invention are generallyprepared using known techniques by reacting starch with an appropriatecross-linking agent in aqueous solution under alkaline conditions. Thedesired cross-linked starches will have a specified relatively lowdegree of cross-linking defined by Brabender viscosity as describedearlier. The amount of cross-linking agent used to achieve this degreeof cross-linking will vary somewhat depending of the conditions andmaterials used. Typically, the amount of cross-linking agent used isfrom about 0.05% to 0.15%, and preferably about 0.1%, by weight of thestarch.

In addition to using the selectively cross-linked starches as definedherein, it has been found that pregelatinizing the starches using aspray-drying process provides a product which has enhanced properties.It is believed that the spray-dried starches possess more uniformparticle size which leads to more uniform and controlled swelling. Theuse of the spray-dry pregelatinization methodology produces starch thatpossesses uniform particle size without the often significantdegradation that occurs when drying and gelatinizing by drum-drying orextrusion methods.

Pregelatinization of the cross-linked starches of this invention may beaccomplished by spray-drying using a steam-injection/dual- orsingle-atomization process described in U.S. Pat. Nos. 4,280,851,4,600,472, or 5,149,799, the disclosures of which are incorporated byreference herein. In this process, a mixture of the granular starch iscooked or gelatinized in an atomized state. The starch which is to becooked is injected through an atomization aperture in the nozzleassembly into the spray of atomized steam so as to heat the starch to atemperature effective to gelatinize the starch. An enclosed chambersurrounds the atomization and heating medium injection apertures anddefines a vent aperture positioned to enable the heated spray of starchto exit the chamber. The arrangement is such that the lapsed timebetween passage of the spray of starch through the chamber, i.e. fromthe atomization chamber-and through the vent aperture, defines thegelatinization time of the starch. The resulting spray-driedpregelatinized starch comprises uniformly gelatinized starch in the formof indented spheres, with a majority of the granules being whole andunbroken and which swell upon rehydration. Nozzles suitable for use inthe preparation of these starches are described in U.S. Pat. No.4,610,760 which is incorporated by reference herein.

The steam injection/dual atomization process as referred to above may bemore particularly described as pregelatinization of the starch by:

a) mixing the starch in an aqueous solvent,

b) atomizing the mixture with an enclosed chamber, and

c) interjecting a heating medium into the atomized mixture in theenclosed chamber to cook the starch, the size and shape of the chamberbeing effective to maintain the temperature and moisture control of thestarch for a period of time sufficient to cook said starch.

A steam injection/single atomization process for cooking andspray-drying starch is disclosed in the U.S. Pat. No. 5,149,799 patentreferred to above and comprises:

a) slurrying the starch in an aqueous medium,

b) feeding a stream of the starch slurry at a pressure from about 50 toabout 250 psig into an atomizing chamber within a spray nozzle,

c) injecting a heating medium into the atomizing chamber at a pressurefrom about 50 to about 250 psig,

d) simultaneously cooking and atomizing the starch slurry as the heatingmedium forces the starch through a vent at the bottom of the chamber,and

e) drying the atomized starch.

It is further noted that blends of the selected cross-linked starchesmay be used. For example, a blend of epichlorohydrin cross-linked starchand phosphorus oxychloride cross-linked starch may be used. Theproportions of the two cross-linked starches are not limited butgenerally a weight ratio of about 4:1 to about 1:4 of epichlorohydrincross-linked starch to phosphorous oxychloride cross-linked starch isused. Preferably, the blend comprises a mixture of about 1:1, by weight,of the starches. The blends of the epichlorohydrin and phosphorusoxychloride cross-linked starches may be prepared by dry-mixing theseparately prepared, spray-dried starches. Alternatively, the blends maybe prepared by simultaneously spray-drying wet mixtures of thecross-linked starches.

The cross-linked starches of the present invention are employed insubterranean treatment fluids in an effective amount to provide fluidloss control and educe fluid loss over a broad temperature range. Theeffective amount of cross-linked starches will vary depending on theother components of the subterranean treatment fluid, as well as thegeological characteristics and conditions of the subterranean formationin which it is employed. Typically, the cross-linked starch fluid losscontrol additive may be used in an amount of from about 1 pound to about10 pounds (lbs) of starch per barrel (bbl) of the subterranean treatmentfluid, preferably from about 3 to about 6 pounds per barrel. The term“barrel” as used herein means a barrel that contains 42 U.S. gallons offluid.

In addition to the cross-linked starches, the subterranean fluids maycontain other components such as a base fluid and often a viscosifyingagent. The base fluid may be an aqueous system containing fresh water,seawater and/or brine. Brine is an aqueous saline solution containingsoluble salts of potassium, sodium, calcium, zinc, and/or cesium and thelike. The viscosifying agent may be xanthan gum, guar gum, otherpolymers and/or clays such as bentonite and/or mixtures of these andlike materials. Other additives known to be used in these subterraneanfluids include, but are not limited to, corrosion inhibitors, oxygenscavengers, antioxidants, biocides, breakers, surfactants as well asmixtures thereof and the like.

The oxygen scavengers and antioxidants may be added to subterraneantreatment fluids to reduce the deleterious effects of oxygen, i.e., theoxidative degradation of the fluid loss control additive, viscosifyingagent, and/or other additives. Exemplary oxygen scavengers includesodium sulfite, sodium dithionite, potassium metabisulfite, and thelike. Exemplary antioxidants include magnesium oxide, triethanolamine(TEA), tetraethylene pentamine (TEPA), and the like. Addition of oxygenscavengers or antioxidants to subterranean treatment fluids may providefluids possessing enhanced viscosity and fluid loss control properties,such that excellent fluid loss control may be maintained over a broadrange of temperatures.

The amounts or proportions of each of the components and additives usedin the subterranean treatment fluid will vary greatly depending on theintended use and purpose of the treatment fluid as well as thegeological characteristics and conditions of the subterranean formationin which the fluid is employed. However, the amount of base fluidgenerally present in the fluid is about 25% to about 99% by weight ofthe fluid. The viscosifying agent may be present in an amount of about0% to about 20% by weight of the fluid. Other additives, such as thoselisted above, may be present in a treatment fluid generally in an amountof about 0% to about 10% by weight of the fluid.

Subterranean treatment fluids for specific purposes require specialadditives. For instance, drilling fluids may also have weighting agents,such as barite, to control the pressure of the formation. Furtherinformation on the composition of drilling fluids can be found in theFifth Edition (1988) of “Composition and Properties of Drilling andCompletion Fluids” by Darley and Gray, the disclosure of which isincorporated by reference herein. Oil well cement slurries may also beclassified as subterranean fluids and often contain Portland cement,retarders, accelerators and similar products. Weighting agents indrilling fluids and cementing agents in slurries or spacer fluids may beused in amounts up to about 50% or more, by weight of the fluid,depending on the requirements of the geological formation. Furtherinformation on the composition of cement slurries can be found in the1987 SPE Monograph on “Cementing” by D. K. Smith, the disclosure ofwhich is incorporated by reference herein. Acidizing fluids wouldinclude acid, typically in amounts of about 1 to about 37% by weight, toetch the formation. The 1979 SPE Monograph “Acidizing Fundamentals” byWilliams et al., the disclosure of which is incorporated by referenceherein, further describes the uses and composition of acidizing fluids.Similarly, other special purpose additives could be used for otherapplications.

The subterranean treatment fluids of this invention contain thecross-linked starch or starch blend, and any viscosifying agent, basefluid and other additive components, present in such proportions thatare appropriate for the specific well site as determined by thoseskilled in the art. For example, a typical drilling fluid containing thefluid loss control additives of the present invention may be prepared byadmixing 4 pounds of the cross-linked starch of this invention, 0.8pounds of high viscosity polyanionic cellulose, 1.1 pounds of xanthangum and 50 pounds of calcium carbonate into 1 barrel (42 U.S. gallons)of water or brine.

As described above, the cross-linked starch fluid loss additives of thisinvention provide good fluid loss control over a broad temperature rangeand in an environment where salinity, shear and high temperaturetolerance are often required. While the degree of fluid loss is arelative term depending on actual conditions of operation, a fluid lossof less than about 100 g, as shown by the low temperature-low pressure(LTLP) API and the high temperature high pressure (HTHP) API tests asdescribed below, has resulted when using the cross-linked starchadditives of this invention. This level of fluid loss control has beenfound to occur over a broad temperature range of about 20° C. to about150° C. (68° F. to 302° F.) in the moderate to high salinity environmentof sea water or saturated sodium chloride solution, used as base fluids.Addition of oxygen scavengers or antioxidants to subterranean treatmentfluids containing the cross-linked starches of this invention mayprovide enhanced fluid loss control over a wider temperature range, e.g.up to about 160° C. (320° F.). Use of higher concentrations of fluidloss control additive and/or viscosifying agent in the subterraneantreatment fluids of this invention may similarly increase fluid losscontrol at very high temperatures.

The examples which follow are intended as an illustration of certainpreferred embodiments of the invention, and no limitation of theinvention is implied. In these examples, the concentration of reagentsand composition components are expressed as parts by weight, unlessotherwise provided. All temperatures are reported in degrees Celsiusunless otherwise noted.

The following test procedures were used in evaluating cross-linkedstarch fluid loss control additives in accordance with this invention.

Brabender Viscometer Test

A Brabender Visco-Amylo Graph is used in this procedure. This is astandard device, readily available on the open market, and is arecording, rotating cup torsion viscometer that measures and recordsapparent viscosity at fixed temperatures or temperature varied at auniform rate.

The procedure for evaluating the cross-linked starch is as follows:

1) A sample of the cross-linked starch, prior to pregelatinization viaspray-drying, is slurried into a solution containing distilled water andglacial acetic acid (2.06% by weight of total charge) to 6.0% anhydroussolids content by total weight,

2) The sample is transferred to the Brabender cup. The cup is theninserted into the viscometer,

3) The glass/mercury thermoregulator is set at 92° C. (198° F.) and thesample is heated at a rate of four degrees per minute to 92° C. Thesample is is then held at about 92° C. until the sample reaches the peakviscosity, and

4) The peak viscosity is recorded. Also recorded is the time, inminutes, that it takes for the sample to reach peak viscosity after itreaches 92° C. (that is, the total time the sample is at 92° C. untilthe sample reaches peak viscosity).

Fluid Loss Testing Procedure

Fluid Preparation

The starch fluid loss control additives were tested in two aqueoussystems: seawater and 26% (w/w; saturated) NaCl brine. The seawater wasprepared by dissolving 18.88 g of dry “Sea-Salt” (ASTM D-1141-52, LakeProducts Company, Maryland Heights, Mo.) into 450 g prepared tap water(the prepared tap water is deionized water containing 1000 ppm NaCl and110 ppm CaCl₂). The 26% NaCl base fluid was prepared by dissolving 141.4g of NaCl into 398.6 g of deionized water.

Prior to salt addition, prepared tap water or deionized water was addedto Hamilton Beach malt mixing cups and mixed at approximately 4000 rpmwith a Hamilton Beach malt mixer. A 1.1 lb/bbl amount of xanthan gum(1.43 g XCD, a product of NutraSweet Kelco Co., a unit of MonsantoCompany, St. Louis. Mo.) was added into each mixing cup and allowed tomix for approximately 3-5 minutes. One drop of 5 M potassium hydroxidewas added to each mixing cup to raise the pH to between 8.5-9 and themixture mixed for 20 minutes at 11,000±200 rpm. At the end of the 20minutes of mixing, the appropriate amount of either “Sea-Salt” or NaClwas added and the fluid was mixed an additional 10 minutes at 11,000rpm.

A 0.8 lb/bbl amount of AquaPAC®—Regular, which is a high viscositypolyanionic cellulose used as a viscosity and filtration control aid(1.07 g; a product of Aqualon Co., Houston, Tex.) and a 4 lb/bbl starchsample (5.14 g), prepared as described below, were dry blended togetherwith a spatula, then added to the fluid mixture. Mixing was continued at11,000 rpm for minutes. The mixing container was removed from the mixerand 50 lb/bbl CC-103 (64.29 g, calcium carbonate, a product of the ECCInternational Co., Sylacauga, Ala.) was added. The mixing cup wasreturned to the mixer and mixed for an additional 5 minutes at 11,000rpm. Octanol (two drops, defoamer) was added and the resulting mixturewas mixed for an additional minute. Finally, the pH of the fluid wasadjusted with 5 M potassium hydroxide to obtain a pH between 8.5 and 9.

Low Temperature/Low Pressure (LTLP) API Fluid Loss Test Procedure

Un-aged samples of the fluid prepared above were tested for fluid lossusing a standard American Petroleum Institute (API) low temperature-lowpressure (LTLP) Fluid Loss Test at room temperature (72° F./22° C.).

Samples of test fluid (300 ml.) were re-mixed using a Hamilton BeachMixer for approximately 1 minute at 11,000 rpm, then poured into an APIFluid Loss filter cell (Fann Instrument Company, Houston, Tex., Model12B, No. 30501) to about a half-inch from the top of the cell. An O-ringand Wattman 50 filter paper were placed in the cell prior to sealing thecell.

The API LTLP Fluid Loss Test was performed at room temperature asfollows. The cell was placed on a filter press, pre-set at 100 psi usingnitrogen pressure, and pressurized for 30 minutes. Fluid lost from thepressurized cell was collected in a tared beaker and weighed.

High Temperature/High Pressure (HTHP) Fluid Loss Test Procedure

Prior to conducting the HTHP API fluid loss test, the samples were agedfor 16 hours at elevated temperatures, as follows.

Heat Rolling Procedure

The fluid containing the test starch sample was poured into a 260 ml.high-temperature aging cell (Fann Instrument Co., Houston, Tex., PartNo. 76000). The cell is made of stainless steel. The fluid filled thecell to approximately one-quarter inch from the top of the cell. Thecell was capped and the outlet cap was screwed on. The cell waspressurized to about 150-200 psi and then the valve stem was carefullytightened. The cell was then placed in the roller oven (Fann InstrumentCo., Houston, Tex., Part No. 7000) that had been preheated to the testtemperature. The roller oven is a standard API roller oven except thatEurotherm temperature controllers (Eurotherm Corp., Reston, Va., Model808) were added to reduce the temperature variance during aging. Thecell was rolled at the test temperature for 16 hours (overnight). Thesample was removed from the oven, cooled to room temperature,depressurized, then tested for high temperature/high pressure (HTHP)fluid loss as described below.

High Temperature/High Pressure (HTHP) API Fluid Loss Test

The cooled sample was placed in a cool 175 ml HTHP fluid loss cell (FannInstrument Co., Part No. 38750) containing a Wattman 50 (or equivalent)filter paper. The bottom valve stem of the cell was closed to preventloss of the fluid prior to heat up. The top cap was attached and thecell placed in a preheated cell holder. A nitrogen pressure line wasattached to the top valve stem and the cell was pressurized toapproximately 200 psi to prevent boiling of the fluid during heat up.Once the cell reached temperature, a condenser was added to the bottomvalve stem of the cell and a back pressure of 100 psi nitrogen pressurewas added to the condenser. The bottom valve stem of the cell was thenopened to allow fluid loss to occur and the pressure of the top valvestem was increased to 600 psi (to provide 500 psi differentialpressure). Fluid loss was measured over a 30 minute time period or untilcomplete fluid loss occurred, whichever comes first. The fluid loss wasmeasured by weight. The fluid loss reported was exactly two times thefluid los s collected (as per API procedures) to compensate for thesmaller surface area of the filter paper compared to the lowtemperature, low pressure fluid loss cell.

Differences between LTLP and HTHP Testing

Testing was conducted as per “API Recommended Practice, StandardProcedure for Field Testing Water-Based Drilling Fluids,” API RP 13B-1,First Edition, Jun. 1, 1990. Room temperature (72° F./22° C.) fluid losstests were conducted using the API low-temperature/low-pressure (LTLP)test procedure (API Proc. RP 13B-l Sect. 3.3). All fluid loss testingabove room temperature was done using the APIhigh-temperature/high-pressure (HTHP) test procedure (API Proc. RP 13B-1Sect. 3.5). The HTHP testing uses different equipment than the LTLP testwhich allows for heating of the filter press and higher differentialpressures. The HTHP testing uses 500 psi differential pressure whereasthe LTLP apparatus uses 100 psi differential. Also, the HTHP uses filterpaper that is one-half the surface area of the LTLP test and, therefore,the fluid loss reported for HTHP testing is doubled that collected.

EXAMPLE 1 Preparation and Testing of Epichlorohydrin Cross-Linked Starch

At room temperature, 1000 g of waxy maize starch was slurried in 1500 gof water. To the slurry, sodium hydroxide, as a 3% solution, was slowlyadded to a pH of about 12.0 (25 ml. of reaction slurry should require18-20 ml. of 0.1 N HCl to neutralize at the phenolphthalein end point).Epichlorohydrin (0.13% by weight) was added to the slurry.

The reaction mixture was allowed to react at 40° C. for hours cooled toroom temperature, and neutralized to a pH of 6.0 with 10-30% solution ofhydrochloric acid. The starch was then filtered, washed and dried toprovide an ungelatinized dry powder. A sample of the cross-linked starchwas analyzed to determine its peak viscosity using a C. W. BrabenderVisco-Amylo Graph, as described above, and found to have a peakviscosity of 1020 Brabender units after 52 minutes at 92° C.

The dried cross-linked starch was slurried in water to 20-30% anhydroussolids by weight. The starch was spray-dried to pregelatinize, using theprocess described above, and in U.S. Pat. Nos. 4,280,851 and 4,600,472.

The resulting dried, pregelatinized powder was tested for fluid lossusing both the API LTLP fluid loss test (room temperature of 72° F./22°C.) and the API HTHP test, described above. The test was in bothseawater and saturated NaCl solution (26%) and gave the results shownbelow in Tables 1 and 2.

TABLE 1 Epichlorohydrin Cross-Linked Starch/Sea Water Fluid LossTemperature (° F.) Fluid Loss (g)  72 (22° C.) 6.4 100 (38° C.) 6.9 150(66° C.) 12.5 175 (80° C.) 18.5 225 (107° C.) 46.1 250 (121° C.) 45.7270 (132° C.) 57.0 290 (143° C.) 64.8

TABLE 2 Epichlorohydrin Cross-Linked Starch/NaCl Solution¹ Fluid LossTemperature (° F.) Fluid Loss (g)  72 (22° C.) 4.9 100 (38° C.) 6.9 150(66° C.) 9.1 175 (80° C.) 21.3 225 (107° C.) 48.7 250 (121° C.) 64.7 270(132° C.) 55.0 290 (140° C.) 15.3 ¹Saturated aqueous NaCl (26%) solution

EXAMPLE 2 Preparation and Testing of Phosphorus Oxychloride Cross-LinkedStarch

At room temperature, 1000 g of waxy maize starch was slurried into anaqueous solution of sodium chloride (1500 g water, 0.5W NaCl by weightof starch). To this slurry, a 3% solution of sodium hydroxide was slowlyadded to a pH of about 12.0 (25 ml. of reaction slurry should require16-18 ml. of 0.1 N HCl to neutralize at the phenolphthalein end point).Phosphorus oxychloride (0.1) was added and the reaction mixture allowedto react for 35 minutes. The resulting reaction mixture was neutralizedto a pH of 6.0 with a 10-30% solution of hydrochloric acid. The starchwas then filtered, washed and dried. A sample of the cross-linked starchwas analyzed to determine its peak viscosity using a C. W. BrabenderVisco-Amylo Graph and found to have a peak viscosity of 1000 Brabenderunits after about 40 minutes at 92° C. The cross-linked starch wasspray-dried and tested for fluid loss as in Example 1 with the resultsshown below in Tables 3 and 4.

TABLE 3 Phosphorus Oxychloride Cross-Linked Starch/Sea Water Fluid LossTemperature (° F.) Fluid Loss (g)  72 (22° C.) 6.2 250 (121° C.) 8.1 260(127° C.) 23.9

TABLE 4 Phosphorus Oxychloride Cross-Linked Starch/NaCl Solution¹ FluidLoss Temperature (° F.) Fluid Loss (g)  72 (22° C.) 5.3 250 (121° C.)21.0 260 (127° C.) 75.6 ¹Saturated aqueous NaCl (26%) solution

EXAMPLE 3

A blend (1:1 wt. ratio) of epichlorohydrin (epi) cross-linked starch andphosphorus oxychloride cross-linked starch (both prepared as in Examples1 and 2) was made and tested for fluid loss in sea water and saturatedNaCl solutions as in the previous Examples. The results are shown belowin Tables 5 and 6.

TABLE 5 Blend of Epi/Phosphorus Oxychloride Cross Linked Starches (1:1)in Sea Water Fluid Loss Temperature (° F.) Fluid Loss (g)  72 (22° C.)6.9 100 (38° C.) 6.1 150 (66° C.) 15.7 250 (121° C.) 33.8 290 (143° C.)47.9

TABLE 6 Blend of Epi/Phosphorous Oxychloride Cross-Linked Starches (1:1)in NaCl Solution¹ Fluid Loss Temperature (° F.) Fluid Loss (g)  72 (22°C.) 5.0 100 (38° C.) 6.3 150 (66° C.) 9.7 250 (121° C.) 31.0 ¹Saturatedaqueous NaCl (26%) solution

Other variations or modifications, which will be obvious to thoseskilled in the art, are within the scope and teachings of thisinvention. This invention is not to be limited except as set forth inthe following claims.

We claim:
 1. A method for controlling fluid loss of a subterraneantreatment fluid in a subterranean formation, said method comprising thestep of introducing the subterranean treatment fluid into thesubterranean formation, wherein said subterranean treatment fluidincludes a fluid loss control additive comprising a pre-gelatinizedcross-linked starch that provides fluid loss control over a temperaturerange of from about 20° C. to about 160° C. and prior topre-gelatinization, said starch has a Brabender peak viscosity of fromabout 800 to about 1250 Brabender viscosity units after about 40 toabout 70 minutes at about 92° C. when subjected to a Brabenderviscometer test.
 2. The method according to claim 1, wherein thesubterranean treatment fluid is a drilling fluid, a workover fluid or acompletion fluid.
 3. The method according to claim 1, wherein the starchis cross-linked with an agent selected from the group consisting ofepichlorohydrin, phosphorus oxychloride, adipic-acetic anhydride andsodium trimetaphosphate.
 4. The method according to claim 3, wherein thestarch is selected from the group consisting of corn, waxy maize,potato, wheat and tapioca.
 5. The method according to claim 4, whereinthe starch is waxy maize.
 6. The method according to claim 5, whereinthe cross-linked starch has a Brabender peak viscosity of from about 920to about 1150 Brabender units after about 40 to about 70 minutes atabout 92° C.
 7. The method according to claim 6, wherein thecross-linked starch exhibits a fluid loss of less than about 100 g whensubjected to a low-temperature-low pressure (LTLP) or high temperaturehigh pressure (HTHP) American Petroleum Institute Fluid Loss Test over atemperature range of from about 20° C. to about 160° C.
 8. The methodaccording to claim 1, wherein the starch is cross-linked withepichlorohydrin and the starch is waxy maize.
 9. The method according toclaim 7, wherein the starch is cross-linked with epichlorohydrin and thestarch is waxy maize.
 10. The method according to claim 1, wherein thecross-linked starch is present in an amount of from about 1 pound perbarrel to about 10 pounds per barrel of subterranean treatment fluid.11. The method according to claim 10, wherein the cross-linked starch ispresent in an amount of from about 3 pounds per barrel to about 6 poundsper barrel of subterranean treatment fluid.
 12. The method according toclaim 10, wherein the cross-linking agent is epichlorohydrin and thestarch is waxy maize starch.
 13. The method according to claim 1,wherein the cross-linked starch is about a 4:1 to about a 1:4 by weightblend of epichlorohydrin cross-linked starch and phosphorus oxychloridecross-linked starch.
 14. The method according to claim 13, wherein thecross-linked starch is about a 1:1 by weight blend of epichlorohydrincross-linked starch and phosphorus oxychloride cross-inked starch. 15.The method according to claim 13, wherein the starch in theepichlorohydrin cross-linked starch and the phosphorus oxychloridecross-linked starch is waxy maize starch.
 16. The method according toany of claims 1-15, wherein the cross-linked starch is pregelatinized byspray-drying.